In a photovoltaic system, an electric power generated by a solar cell is influenced by weather conditions and the like, and a voltage is changed by a temperature variation in the solar cell itself and the like. Therefore, a secondary battery is used as a backup power source, and when the amount of the electric power generated by the solar cell is small, the electric power is discharged from the secondary battery to stabilize the electric power supplied to a load. The voltage of the secondary battery is set to be slightly lower than the voltage at which the electric power generation of the solar cell is stable and is charged from the solar cell when the electric power consumed by the load is small.
In a hybrid power source including a solar cell, a secondary battery for backup of the solar cell and the like, since the voltage of the solar cell is different from the rated voltage of the load and the secondary battery, the voltage is stepped up/down by using a DC/DC converter to supply an electric power to the load. In the general circuit configuration, a DC/DC converter is provided for each of the solar cell and the secondary battery.
Therefore, in designing the DC/DC converter, it is important to achieve both the miniaturization and high efficiency of the DC/DC converter, and it has been proposed that a plurality of DC power sources share a transformer and a rectifier circuit (see, e.g., JP 2005-2297, herein after Patent Document 1).
Patent Document 1 does not disclose a specific circuit configuration of the DC/DC converter, but FIG. 12 shows a circuit configuration of a DC/DC converter 50 using a general MOSFET as a switch element. A first and a second primary winding N51 and N52 corresponding to a first and a second DC power source 51 and 52, respectively, are provided on a primary side of a transformer 53, and one secondary winding N53 corresponding to the load 57 is provided on the secondary side of the transformer 53. A first oscillation circuit 54 having a full-bridge structure, which is formed of four switch elements Q51 to Q54, is connected to the first primary winding N51. Further, a second oscillation circuit 55 having a full-bridge structure, which is formed of four switch elements Q55 to Q58, is connected to the second primary winding N52. A rectifier circuit 56 is connected to the secondary winding N53.
The first DC power source 51 is a solar cell and the second DC power source 52 is a secondary battery. The voltage of the solar cell is VDC1, and a reference voltage of the solar cell is Vref1. The voltage of the secondary battery is VDC2, and a reference voltage of the secondary battery is Vref2. The number of turns of the first primary winding N51 and the number of turns of the second primary winding N52 are n1 and n2, respectively.
In order that the output voltage of the load by the discharge operation from the secondary battery and the power generation of the solar cell is kept constant, it is preferable to set a turns ratio n2/n1 of the primary windings N51 and N52 to satisfy Vref1×n2/n1=Vref2.
However, considering that the secondary battery is charged from the solar cell, it is preferable to set a turns ratio n2/n1 of the primary windings N51 and N52 to satisfy Vref1×n2/n1>Vref2.
In practice, since the voltage applied to the load 57 is not constant and has a tolerance value, it is set to satisfy Vref1×n2/n1>Vref2. However, in order to more easily describe the nature of the problem in the present invention, the problem will be described below on the assumption that a turns ratio satisfies Vref1×n2/n1=Vref2.
FIG. 13 shows a state where in the case of VDC1×n2/n1>VDC2, for example, under the condition that the voltage of the solar cell is varied to be larger than the reference voltage Vref1 and the voltage of the secondary battery is the reference voltage Vref2 (VDC1>Vref1, VDC2=Vref2), the switch elements Q55 to Q58 are turned off while the switch elements Q51 and Q54 and the switch elements Q52 and Q53 are alternately turned on and off, so that the electric power is supplied to the load 57 from the first DC power source 51. In FIG. 13, the switch elements Q51 and Q54 are being turned on. When supplying the electric power to the load 57 from both the first and the second DC power source 51 and 52, the switch elements Q51 and Q54, the switch elements Q52 and Q53, the switch elements Q55 and Q58, and the switch elements Q56 and Q57 may be sequentially turned on by time division.
Under the conditions of VDC1>Vref1 and VDC2=Vref2, the voltage VN52 of the primary winding N52 satisfies VN52=VDC1×n2/n1=VDC1×Vref2/Vref1>VDC2 by the induced electromotive force generated in the primary winding N52 from the primary winding N51. Thus, the voltage of the primary winding N52 becomes larger than VDC2. Since MOSFET has a body diode (parasitic diode), a reverse current flows through the second DC power source 52 by the electromotive force generated in the second primary winding N52 via the body diodes of the switch elements Q55 and Q58. The same is true when the switch elements Q52 and Q53 are turned on. Since such a reverse current becomes a charging current to the secondary battery, substantially, the first DC power source 51 charges the secondary battery while supplying the electric power to the load 57, thereby resulting in an increase in the current flowing through the first oscillation circuit 54 on the side of the first DC power source 51.
Accordingly, the loss due to the switch elements Q51 to Q54 included in the first oscillation circuit 54 is increased, and the power supply efficiency of the first DC power source 51 is decreased. Further, the secondary battery is charged through the body diodes of the switch elements Q55 to Q58, and there occurs a problem such that it cannot be charged at certain timings (even if charging is not desired, it is charged arbitrarily).
Similarly, under the condition of VDC1×n2/n1<VDC2, when the electric power is outputted from the second DC power source 52, a reverse current flows through the first DC power source 51. That is, in the configuration of FIG. 13, the voltage of the first and the second DC power source 51 and 52 varies, and there occurs a problem such that the efficiency is deteriorated in the case of VDC1×n2/n1≠VDC2.
In another conventional example shown in FIG. 14, in order to prevent the reverse current from flowing through the first DC power source 51 or the second DC power source 52, backflow prevention diodes D51 to D58 are connected in series to the switch elements Q51 to Q58, respectively, in opposite directions to the body diodes (see Chen et al. “Multi-Input DC/DC Converter Based on the Flux Additivity,” herein after Non-patent Document 1).
However, when a current flows in the forward direction through the backflow prevention diodes, the loss due to the diodes is increased and the power supply efficiency from the first DC power source 51 or the second DC power source 52 is decreased. Further, it is necessary to add the backflow prevention diodes D51 to D58 or choke coils C51 and C52 to the oscillation circuits 54 and 55, which results in reducing an advantage of the miniaturization of the DC/DC converter 50 obtained by sharing the transformer 53 and the rectifier circuit 56. In addition, since the reverse current does not flow through the second DC power source 52 by the backflow prevention diodes, the secondary battery cannot be charged by using this DC/DC converter.